The present invention relates generally to rotary hook drive devices for sewing machines including a pivoting-type loop catcher called a “semi rotary hook”, and particularly to multi-head sewing machines (embroidery sewing machines and other types of sewing machines) including a plurality of sewing heads and a plurality of hook drive devices provided in corresponding relation to the sewing heads.
The rotary hooks, which are means for catching a lower thread in a sewing machine, come in two major types: a full rotary type; and a semi rotary type. The semi rotary (type) hook is used, for example, when a thick sewing work piece is to be sewn. Japanese Patent Application Laid-open Publication No. 2001-104671 (hereinafter referred to as “patent literature 1”) discloses a semi rotary hook drive mechanism, where a single common drive shaft is rotatably supported by individual hook support members. The drive shaft is connected in such a manner that its driving force is transmitted to the semi rotary hooks within the individual hook support members. The drive shaft is also connected to a main shaft of the sewing machine via a drive conversion mechanism, so that it reciprocatively pivots within a predetermined angular range as the main shaft of the sewing machine pivots in one direction. Such reciprocating pivotal movement of the drive shaft causes respective rotary hook shafts to reciprocatively pivot so that the semi rotary hooks pivot.
According to the disclosure of patent literature 1, the drive shaft provided in common to the individual hook support members is caused to reciprocate pivotal movement in forward and reverse directions. Therefore, in a case where many machine heads are employed and thus the drive shaft has a large length, and when the direction of the pivotal movement of the drive shaft is changed, great torsion would be produced in a free end portion, remote from the drive conversion mechanism, of the drive shaft due to rotational inertia; consequently, the free end portion would pivot through more than a predetermined angle and thus the semi rotary hooks would pivot more than necessary. As a consequence, the semi rotary hooks would be driven at improper timing deviated from required timing, which may not only cause breakage of threads but also cause the semi rotary hooks and sewing needles to contact with each other so that they are undesirably impaired.
Korean Utility Model Registration No. 20-405210 (hereinafter referred to as “patent literature 2”) discloses a semi rotary hook drive mechanism, which rotates a rotary hook drive shaft in one direction so as to pivot semi rotary hooks via drive conversion mechanisms provided in respective rotary hook support members. The semi rotary hook drive mechanism employed in a first embodiment disclosed in patent literature 2 is constructed in such a manner that the driving force of the drive shaft is transmitted via gear connections to a crank mechanism. The thus-transmitted driving force is converted by the crank mechanism into reciprocative pivoting movement (force), which is then transmitted via gear connections to rotary hook shafts to pivot the semi rotary hooks. Because the drive shaft is rotated in one direction in the disclosed semi rotary hook drive mechanism, the torsion produced due to rotational inertia can be reduced and deviation of the driving timing of the rotary hooks can also be reduced, even where many heads are employed. As a result, stable sewing can be achieved. However, because the drive conversion mechanism, including the crank mechanism and two sets of the gear connections, has an increased number of necessary components, which would increase the cost and size of the semi rotary hook drive mechanism and impose a great load on the drive shaft due to the increased size.
As set forth above, in the semi rotary hook drive mechanism disclosed in patent literature 1, where the drive shaft reciprocatively pivots in the forward and reverse directions, great torsion is produced in the free end portion, remote from the drive conversion mechanism, of the drive shaft, and thus, the semi rotary hooks would pivot more than necessary. Consequently, deviation in the driving timing (and hence untimely driving) of the semi rotary hooks would occur, which may not only prevent stable sewing but also cause damage or impairment of the semi rotary hooks and sewing needles. Further, the semi rotary hook drive mechanism disclosed in patent literature 2, where the drive shaft is rotated in one direction, would require an increased number of necessary components and impose a great load on the drive shaft although it can reduce deviation in the driving timing of the semi rotary hooks.